Ukrainians abandoned inundated homes on Wednesday as floods crested across the south after the destruction of a huge hydroelectric dam on front lines between Russian and Ukrainian forces, with their presidents trading blame for the disaster.

Residents slogged through flooded streets carrying children on their shoulders, dogs in their arms and belongings in plastic bags while rescuers used rubber boats to search areas where the waters reached above head height.

Ukraine said the deluge would leave hundreds of thousands of people without access to drinking water, swamp tens of thousands of hectares of agricultural land and turn at least 500,000 hectares deprived of irrigation into "deserts".

Ukrainian President Volodymyr Zelenskiy said in a video address that it was impossible to predict how many people would die in Russian-occupied areas due to the flooding, urging a "clear and rapid reaction from the world" to support victims.

"The situation in occupied parts of the Kherson region is absolutely catastrophic. The occupiers are simply abandoning people in frightful conditions. No help, without water, left on the roofs of houses in submerged communities," he said.

Visiting the city of Kherson downstream from the dam, Deputy Prime Minister Oleksandr Kubrakov said over 80 settlements had been affected by the disaster, and that the flooding had released chemicals and infectious bacteria into the water.

The Nova Kakhovka dam collapse on Tuesday happened as Ukraine prepares a major counteroffensive, likely the war's next major phase. Both sides traded blame for continued shelling across the populated flood zone and warned of drifting landmines unearthed by the flooding.

Kyiv said on Wednesday its troops in the east had advanced more than a kilometre around the ruined city of Bakhmut in eastern Ukraine, its most explicit claim of progress since Russia reported the start of the Ukrainian counteroffensive earlier this week. Russia said it had fought off the assault.

Kyiv said several months ago the dam had been mined by Russian forces that captured it early in their 15-month-old attack, and has suggested Moscow blew it up to try to prevent Ukrainian forces crossing the Dnipro in their counteroffensive.

Russian President Vladimir Putin accused Ukraine of destroying the dam at the suggestion of Western supporters, saying it was a "barbaric" war crime that escalated the conflict with Moscow. Putin described the incident as an "environmental and humanitarian catastrophe", according to a Kremlin read-out.

Neither side has presented public evidence demonstrating who was responsible. Some experts say the dam may have collapsed due to earlier war damage and poor Russian management. (Reuters)